A clean-up for the previous patch

mysql-test/r/ctype_utf8.result

@@ -5954,6 +5954,13 @@ SET @arg00=_binary 0xFF;
 EXECUTE stmt USING @arg00;
 ERROR HY000: Invalid utf8 character string: 'FF'
 DEALLOCATE PREPARE stmt;
+SET NAMES latin1;
+PREPARE stmt FROM "SELECT CONCAT(_utf8'a' COLLATE utf8_unicode_ci, ?)";
+EXECUTE stmt USING @no_such_var;
+CONCAT(_utf8'a' COLLATE utf8_unicode_ci, ?)
+NULL
+DEALLOCATE PREPARE stmt;
+SET NAMES utf8;
 #
 # End of 10.0 tests
 #

mysql-test/t/ctype_utf8.test

@@ -1675,6 +1675,11 @@ SET @arg00=_binary 0xFF;
 --error ER_INVALID_CHARACTER_STRING
 EXECUTE stmt USING @arg00;
 DEALLOCATE PREPARE stmt;
+SET NAMES latin1;
+PREPARE stmt FROM "SELECT CONCAT(_utf8'a' COLLATE utf8_unicode_ci, ?)";
+EXECUTE stmt USING @no_such_var;
+DEALLOCATE PREPARE stmt;
+SET NAMES utf8;
 
 --echo #
 --echo # End of 10.0 tests

sql/item.cc

@@ -1274,6 +1274,8 @@ Item *Item_param::safe_charset_converter(CHARSET_INFO *tocs)
   */
   if (const_item())
   {
+    if (state == NULL_VALUE)
+      return this;
     uint cnv_errors;
     String *ostr= val_str(&cnvstr);
     if (!needs_charset_converter(tocs))